484 A TEXTBOOK OF PHYSIOLOGY 



cerned. As indicated, the action of these nerves varies in different 

 animals. It is possible that both are usually concerned in the act 

 of micturition, particularly in raising the tension for the initial pro- 

 cess. The pelvic nerves, when stimulated peripherally, usually cause 

 a marked contraction of the body of the bladder, and an inhibition 

 of the sphincter of the trigone, while the hypogastrics cause an 

 inhibition of the wall of the bladder. The latter are therefore 

 mainly in action during the accumulation of urine within the bladder ; 

 the pelvic, on the other hand, during the voiding of the viscus. 



The hypogastric supply of the bladder affords an example of what 

 is known as an " axon -reflex." If all the nerves connected wdth the 

 inferior mesenteric ganglion be divided with the exception of the 

 right hypogastric nerve to the bladder, then stimulation of the central 

 end of the left hypogastric nerve will cause a contraction of the right 

 half of the bladder. The explanation is that the pregaiiglionic fibre 

 branches in the ganglion, one branch forming a cell station with the 

 right nerve, another branch continuing in the left nerve to the bladder. 

 Stimulation of the left nerve therefore can influence the bladder through 

 the cell station in the ganglion. 



The last drops of urine are expelled from the urethra by the con- 

 tractions of the bulbo-cavernosus (accelerator urinae) muscles. The 

 act of micturition can be stopped by the contraction of the com- 

 pressor urethrse, but it is difficult to do this when the reflex is in full 

 action. 



